1. Field of the Invention
The present invention relates to a speed change system for a working vehicle comprising a multistage transmission which changes the speed of prime mover power outputted from a prime mover among plural speed stages.
2. Related Art
As disclosed in JP 2003-314679 A, there is a well-known speed change system used for a working vehicle such as a tractor that has a multistage main transmission comprising a plurality of gear trains. In this speed change system, to improve the speed change operability of the main transmission, the main transmission includes two mechanical clutches such that, during the speed change operation, the engaging action of one clutch overlaps the disengaging action of the other clutch so as to realize a double clutch-engaging state. This allows an operator to shift gears without interrupting the transmission of rotary power from a prime mover, such as an engine or an electric motor, to an axle, thereby reducing the shock and noise caused by the gearshift.
To enable a wide range of speeds—from low speeds suitable for activities such as forming a ridge to high speeds suitable for traveling—the working vehicle includes a sub transmission in addition to the main transmission so as to increase the number of speed stages. In other words, multiple speed stages are enabled by the combination of the main transmission and the sub transmission, thereby ensuring the ability to fine-tune speed changes over a broad range of speeds. However, the speed change operation of the sub transmission cannot be performed during traveling of the working vehicle; it can be performed only when the working vehicle is stationary. While the working vehicle is stationary, an operator selects the sub speed stage suitable for the work to be performed. After that, the operator operates a brake and an accelerator pedal so as to start the working vehicle. Then, during the traveling of the vehicle, one of the main speed stages suitable for the vehicle speed, i.e., the actual traveling speed of the vehicle, is selected manually or automatically.
Accordingly, while the working vehicle travels at a set speed stage, if an operator desires fine-tuned speed change, the operator can shift up or down the main speed stage by a main speed change operation, while still operating within the same sub speed stage. While still traveling, an operator cannot effect more significant speed changes by selecting other speed stages that depend on the sub speed change operation. To achieve a more significant speed change by selecting a speed stage depending on the sub speed change operation, the working vehicle must be stopped, thereby reducing the operability for speed change, causing frequent start and stop of the vehicle and worsening fuel economy.